The Auxiliary Power Unit (APU) provides auxiliary power (pneumatic and electric) to the various systems in an aircraft and Main Engine Start.
The APU is conventionally suspended by several rigid linkages that attach it to the fuselage, generally in the tailcone structure.
The APU suspension system ensures that all the rigid body degrees of freedom of the system are removed while allowing relative motion to absorb the thermal expansion of the APU and manufacturing and assembly tolerances. Additionally, it has the purpose of isolating the aircraft from the machine vibrations and vice-versa, for instance, isolating the APU from aerodynamic and inertia dynamic loads coming from the aircraft, by creating a spring damper system that avoids dynamic coupling between the APU and the aircraft and minimizes load amplifications at critical frequencies.
Known suspension systems comprise several rigid struts or suspension rods for attaching the APU to the tailcone structure. They also comprise vibration isolators attached to the end of the struts adjacent to the engine or auxiliary power unit for reducing the transmission of vibration and shocks from the APU to the aircraft structure and vice-versa. The vibration isolators comprise a housing in connection with the struts for enclosing an elastomeric element for absorbing the vibrations.
Furthermore, known suspension systems comprise APU attachment brackets for joining the vibration isolators to the APU. The struts are joined to an extension of the housing. Each vibration isolator and its corresponding APU attachment bracket is called a mount.
Finally, several structure attachment brackets are located at the end of the struts adjacent to the fuselage tailcone structure for joining the struts to the fuselage structure.
In this way the rigid struts suspension system links the APU to the fuselage structure rigidly, the struts absorbing tension, compression and, in some design concepts, shear loads.
One example of these systems is the one described in WO 2006/108028 A2, which relates to an aircraft auxiliary power unit suspension system for isolating an aircraft auxiliary power unit including at least one suspension linkage.
However, these systems lead to some problems relating to wearing, resonances and damage on fittings due to the load transmission. Besides that, the housing of the damper systems is usually heavy and not very efficient.